Due to the well known power advantages of MOS integrated circuits, they are a natural choice for certain applications such as space satellites. However, in this and other high radiation dose environments, MOS devices must be specially protected, or radiation hardened, to avoid radiation-induced failure.
The most troublesome mechanism of radiation damage involves the buildup of positive charge in the field oxide near the Si--SiO.sub.2 interface. This charge does not interfere with the performance of P-channel devices, but in N-channel devices it causes an inversion of the substrate or tub surrounding the active regions, providing an unwanted current path.
Well developed techniques exist for growing radiation hard gate oxide, in which control of positive charge accumulation is possible due to the relatively thin layer of oxide needed. The thick field oxide surrounding the active region, however, is not susceptible to these methods of radiation hardening because it is too thick. Therefore, techniques have evolved for increasing the doping level of the customary p-type guard region surrounding each N-channel device to preclude the possibility that radiation-induced inversion layers will extend between two devices under the field oxide.
When this technique is applied to high-density processes referred to as local oxidation of silicon processes, the heavily doped, p-type guard region abuts the n-type source and drain regions, resulting in a drain-to-substrate breakdown voltage which is unacceptably low.